The present invention relates to a process for polishing semiconductors and more particularly to a process for polishing substrates of CdS.
In the fabrication of semiconductor devices, it is essential to prepare the starting material into substrates having flat, smooth damage-free surfaces. A highly polished, extremely clean and flat CdS surface is necessary for a good reflective mode liquid crystal light valve.
Several techniques such as electropolishing, chemical etching and mechanical lapping and polishing surfaces of semiconductor slices are well known and presently in use in the semiconductor art. By lapping and polishing techniques a surface can be prepared which is about 0.0001 inch per inch flat. The average roughness of this surface is typically 0.3 microinch. The resultant surfaces are smooth and flat enough for satisfactory device fabrication but the mechanical damage which is still present on the surface deleteriously affects the electrical characteristics of devices. Typically, it is necessary to remove such damaged surfaces prior to further processing by chemically etching the surfaces. Unfortunately, chemical etching with conventional etchants, such as aqua regia or mixtures of nitric and hydrofluoric acids, increases the average roughness of a typical surface to about 3.0 microinches or greater. Thus, the gain in removing mechanical damage by the chemical etching technique is obtained at the expense of increasing the average roughness.
In U.S. Pat. No. 3,262,825, entitled, "Method For Etching Crystals of Group III(a)-V(a) Compounds and Etchant Used Therefor", which issued July 26, 1966, to Calvin S. Fuller, there is disclosed an etchant and method for polishing substrates of gallium arsenide and gallium phosphide. The etchant is comprised of a non-aqueous mixture of an organic compound selected from among glacial acetic acid and methanol and a material selected from the group consisting of a halogen selected from among chlorine, bromine and iodine.
A method for making haze-free surfaces of semiconductor bodies is disclosed in U.S. Pat. No. 4,070,797, which issued Jan. 31, 1978, to Rudolf Griesshammer and Gunther Lechner. In this patented process, the surface of a substrate is first polished using either quartz, silica, silicates and fluosilicates and a second polishing of the surface is done with one of the named agents to which a nitrogen-free surfactant has been added.